WARNING: No server certificate verification method has been enabled. See http://openvpn.net/howto.html#mitm for more info.My knowledge about OpenVPN isn't really good but I didn't found a possibility in pfsense Cert Manager to get this working:

You can build your server certificates with the build-key-server script (see the easy-rsa documentation for more info). This will designate the certificate as a server-only certificate by setting the right attributes. Now add the following line to your client configuration:

remote-cert-tls server

Did I miss something in the Cert Manager config options or isn't there a possibility in there till now ?